1. Technical Field
The present invention relates generally-to a level shifter circuit and, more particularly, to a level shifter circuit for converting an input signal referenced to ground, or, more generally, the least positive power supply, to an output signal referenced to a higher, more usable voltage.
2. Background Art
In signal processing, it may become necessary to accurately process signals which can take on values close to the power supplies. Unfortunately, such processing requirements can create a problem because the transistors utilized in the signal processing circuits require signal-independent biasing that subtracts from the available supply voltage in determining the extreme limits of acceptable input signals (often. referred to as "headroom"). U.S. Pat. No. 4,767,946, "High-Speed Supply Independent Level Shifter," (issued August 1988 to Taylor and assigned to Tektronix, Inc.) discloses a level shifter which is designed to provide a supply independent input potential V.sub.1 across the drive stage, given a required output potential V.sub.2 to a subsequent stage. The level shifter is incorporated into a first current leg between first and second supply voltages. A second reference current leg is connected in parallel with the first current leg. A current mirror, coupling the first current leg to the second reference current leg, establishes a fixed relationship between the current flowing through each of the current legs. A floating voltage source V.sub.3, which is an additive function of potentials V.sub.1 and V.sub.2, is designed to provide a desired V.sub.1, given V.sub.2, such that V.sub.1 is independent of the supply voltages.
Other examples of level shifting circuits include U.S. Pat. No. 4,743,862, "JFET Current Mirror and Voltage Level Shifting Apparatus," (issued May 1988 to Scheinberg and assigned to Anadigics, Inc); U.S. Pat. No. 4,899,308, "High Density ROM in a CMOS Gate Array," (issued February 1990 to Khan and assigned to Fairchild Semiconductor Corp.); U.S. Pat. No. 4,692,641, "Level Shifting Circuitry for Serial-To-Parallel Converter," (issued September 1987 to Highton and assigned to Burr-Brown Corp.); U.S. Pat. No. 4,714,871, "Level Shifter for a Power Supply Regulator in a Television Apparatus," (issued December 1987 to Craft et al. and assigned to RCA Corp.); and U.S. Pat. No. 4,769,590, "Differential Level Shifter Employing Current Mirror," (issued September 1988 to Taylor and assigned to Tektronix, Inc.). All of the above-referenced patents are hereby incorporated herein by reference.
Although a wide variety of level shifting systems, such as those discussed above, have been developed to convert input and/or output signals to more appropriate, system dependent levels within signal processing circuits, currently available level shifting systems fail to address the problem solved by the instant invention, namely, the conversion of an input signal having a voltage close to the least positive power supply, to an output signal adjustably referenced to a higher, more easily usable voltage.